1. Field of the Invention
The present invention relates to a method of estimating a position (e.g., magnetic pole position) of a rotor in a synchronous motor and an apparatus of controlling the motor based on the estimated position.
2. Description of Related Art
In a synchronous motor such as a surface permanent-magnetic synchronous motor (SPMSM) or an internal permanent-magnetic synchronous motor (IPMSM), permanent magnets are used for a rotor, and S and N magnetic poles formed by the permanent magnets are alternately disposed around a rotor shaft. During the operation of this motor, an alternating current voltage is applied to windings of a stator, and an alternating current generated from the applied voltage flows through the windings. A phase of the voltage is controlled in synchronization with a position (e.g., magnetic pole position) of a rotor. Therefore, a position detector for detecting the magnetic pole position of the rotor is required. As this detector, a Hall element, an encoder or a resolver is used. However, it is difficult to downsize a motor with a position detector, and it is required to connect the detector with a controller for controlling the motor through connection lines.
Therefore, to control a synchronous motor without using any position detector, a magnetic pole position of a rotor in a motor has recently been estimated by using an induced voltage of the motor. During the operation of the motor, the motor itself induces a voltage which is changed with the magnetic pole position. Therefore, the magnetic pole position can be estimated from the induced voltage. For example, Published Japanese Patent First Publication No. 2001-251889 discloses a motor model expression approximated on a dq rotational coordinates system having a d-axis and a q-axis to estimate a magnetic pole position based on an induced voltage. The d-axis is set so as to be directed from an S magnetic pole to an N magnetic pole in a rotor. The q-axis is set to be orthogonal to the d-axis on a plane perpendicular to a rotation axis of the rotor. An original point of the coordinates system is placed on the rotation axis.
In the estimation using this motor model expression, apparatus parameters set at constant values are used for constituting a motor model expression, and a shaft shift Δθ of an estimated pole position from a true pole position (i.e., d-axis) in a synchronous motor is calculated from a component Idc (hereinafter, called d-axis component) of a detected direct current along the d-axis, a component Iqc (hereinafter, called q-axis component) of the detected direct current along the q-axis, a d-axis component V*d of a voltage applied to the motor, a q-axis component V*q of the applied voltage and an instructed rotational speed ω*r of the rotor. An estimated magnetic pole position of the rotor is calculated from the shift Δθ.
However, the parameter values appropriate to an actual motor are varied due to various causes. For example, even though motors are produced in mass production, sizes and shapes of constituting parts of one motor differ from those of the other motors. In this case, a true value of each parameter in a motor is determined in dependent on the sizes and shapes of parts of the motor. Further, the parameter values appropriate to a motor are varied when a magnetic field is saturated in the motor.
To solve this problem, for example, Published Japanese Patent First Publication No. 2005-130691 discloses a voltage formula as a motor model expression. In this model, to minimize an error in an estimated magnetic pole position, a q-axis inductance denoting one of apparatus parameters is renewed, and an estimated magnetic pole position is determined according to a motor model expression using the renewed inductance. More specifically, a first estimated magnetic pole position is calculated according to a motor model expression using an instructed voltage applied to a motor, a current detected from the motor and a changeable q-axis inductance, and a second estimated magnetic pole position is calculated without using any q-axis inductance. The q-axis inductance is renewed based on a difference between the first and second positions, and a magnetic pole position is estimated based on the renewed inductance at a next estimation cycle.
However, in this Publication, because two estimated positions are calculated for each estimation cycle, a large amount of calculation is required to correctly estimate a magnetic pole position.